Analog filters are frequently used in wireless devices such as cellular telephones. However, bandwidth variations in an analog filter may lead to significant performance degradation in both the receive and transmit signal paths of the wireless device. In the receive path, variations in the bandwidth of the receiver's baseband analog filter leads to performance degradation in static sensitivity, sensitivity in the presence of interferers, receiver IP3, and anti-aliasing performance. For example, in the case of wideband code-division multiple access (WCDMA), the 0.1% bit error rate (BER) static sensitivity degradation due to +/−12% analog filter bandwidth variation is around 0.5 dB.
Alternately, in the transmit path, variations in the transmitter's baseband filter bandwidth leads to performance degradation in the transmitter's EVM (Error Vector Magnitude), ACLR (Adjacent Channel Leakage Ratio), and static/transient power mask performance. As an example, greater than 5% variation in the transmit baseband filter's bandwidth leads to significantly reduced margin to be able to meet strict EVM requirements for the EDGE protocol.
One prior art method for baseband analog filter tuning is based on the concept of master-slave tracking. Either a filter stage or a high-Q biquad stage is used as an oscillator with the exact same topology as the circuit used in the sections of the main filter. Any process and/or temperature variations should affect the main filter and the slave circuit by the same amount. Therefore, to insure that the process and temperature variations of the slave circuit matches those of the main filter, the slave circuit is positioned in close proximity to the main filter on the integrated circuit. The next step is to establish a closed control loop, which may be a phase-locked-loop (PLL) around the slave to keep the frequency oscillation of the biquad oscillator (or the phase difference if a slave of the filter is being used) always close to a stable value (i.e. an external crystal oscillator frequency or a predefined phase difference). In so doing, the frequency properties of the filter are stable with respect to process/temperature variations due to the matching between the master/slave circuits. The frequency of oscillation of the biquad or the cutoff frequency of the filter is controlled in the PLL by tuning all the resistors (or capacitors) of the master filter using a binary word resultant from the phase comparison between the stable reference frequency and the slave biquad oscillation.
However, in practice, matching the process and temperature variations between the main filter and the slave circuit is difficult because in some embodiments, the main filter may be very large and complex. Even if the slave circuit is placed directly adjacent to the main filter, it may still be relatively far away from some of the filter sections.
Therefore, there is a need for a more accurate method for controlling the bandwidth frequency of an analog filter.